1. Field of the Invention
The present invention relates to an antenna and an antenna module used for communication utilizing electromagnetic coupling such as RFID communication.
2. Description of the Related Art
In recent years, proximity communication systems using various non-contact ICs have been broadly used in various fields. Such a communication system includes a non-contact IC card including a wireless communication IC and a card reader. In this communication system, when the non-contact IC card is moved closer to the card reader within a predetermined distance, communication is performed. To perform the communication, an antenna in which a resonant frequency is set in accordance with a frequency of a communication signal is required. Such an antenna disclosed in Japanese Unexamined Patent Application Publication No. 2001-84463 and Japanese Unexamined Patent Application Publication No. 10-334203 basically has a coil electrode wound in a planar manner and generates a capacitance used to set a resonant frequency together with an inductance of the coil electrode.
In Japanese Unexamined Patent Application Publication No. 2001-84463, for example, the antenna includes coil electrodes wound on front and back surfaces of an insulation sheet in a predetermined manner. These coil electrodes are arranged so as to face each other such that a desired capacitance is generated. Here, the coil electrodes have large widths, and accordingly, a large capacitance is obtained.
Furthermore, in an example of the related art described in Japanese Unexamined Patent Application Publication No. 2001-84463, a coil electrode and one of a pair of counter electrodes of a capacitor are formed on a front surface of an insulation sheet, and the other counter electrode of the capacitor is formed on a back surface. In this configuration, a conductive through hole is mechanically formed in the insulation sheet so that the counter electrode formed on the back surface and a circuit pattern formed on the front surface are connected to each other.
Furthermore, in Japanese Unexamined Patent Application Publication No. 10-334203, a coil electrode is formed on a front surface of an insulation sheet, and an electrostatic capacitance controlling pattern used to generate a capacitance with the coil electrode is formed on a back surface. The capacitance is controlled by controlling a shape (line length) of the electrostatic capacitance controlling pattern.
However, in the configuration disclosed in Japanese Unexamined Patent Application Publication No. 2001-84463 above, since the numbers of windings of the coil electrodes are reduced and the coil electrodes have the large widths, a considerably small inductance is obtained although the large capacitance is obtained. Therefore, a magnetic field which can be radiated from the antenna becomes weak and a communication-available distance becomes small. Accordingly, the configuration is not suitable for data communication which requires a predetermined signal level.
Furthermore, in the configuration disclosed in Japanese Unexamined Patent Application Publication No. 2001-84463, the insulation sheet is mechanically punched through so that the electrode pattern formed on the front surface and the electrode pattern formed on the back surface are brought to a conductive state. Accordingly a fabrication process is complicated.
Moreover, in the configuration disclosed in Japanese Unexamined Patent Application Publication No. 10-334203, the electrostatic capacitance controlling pattern is formed on the back surface in a direction that is the same as a winding direction of the coil electrode formed on the front surface in a plan view, that is, when viewed in a direction along a magnetic field on a surface of the antenna. Accordingly, the electrostatic capacitance controlling pattern formed on the back surface does not contribute to the inductance of the antenna, and the inductance only depends on the pattern of the coil electrode formed on the front surface. Therefore, in order to increase the inductance to strengthen the radiation magnetic field, the number of windings of the coil electrode formed on the front surface should be increased, that is, a large antenna should be configured.